Method and apparatus for removal of condensed deposits from mold covers



May 28, 1968 D. G. FLECK 3,385,346

METHOD AND APPARATUS FOR REMOVAL OF CONDENSED DEPOSITS FROM MOLD COVERS Filed Aug. 26, 1965 A I a if /a x f; S? 9 r f 02% [A ENTOR c Dona/d fleck BY ATT RNEYS United States Patent 3,385,346 METHOD AND APPARATUS FOR REMOVAL OF CONDENSED DEPOSITS FROM MOLD COVERS Donald G. Fleck, Alliance, Ohio, assiguor to TRW Inc., Cleveland, Ohio, a corporation of Ohio Filed Aug. 26, 1965, Ser. No. 482,800 2 Claims. (Cl. 164-121) ABSTRACT OF THE DISCLOSURE Method and apparatus for casting high temperature alloys wherein a ceramic mold is positioned within a high temperature furnace beneath an apertured cover plate through which molten metal is introduced into the mold, and a cover is removably positioned within the aperture, the cover having an angularly inclined inner surface which is at a relatively cooler temperature than the remainder of the furnace so that the volatilized impurities condense thereon and are removed when the cover is removed for the pouring of metal into the mold.

The present invention relates to a method and apparatus for casting high temperature alloys and is particularly concerned with the problem of avoiding contamination of the metal with foreign substances.

A recently developed method for producing superalloy castings involves the directional solidification of the metal within the mold due to the presence of a predetermined thermal gradient. Typically, the casting is made in a relatively porous ceramic type open ended shell mold, positioned within the furnace so that it rests on a chill block composed of copper or other highly heat conductive material. The mold is preheated, typically by an induction coil which surrounds a graphite susceptor forming the inner wall of the furnace itself. The space between the coil and the graphite susceptor is usually filled with some insulating material, and the top of the furnace assembly must be covered with some insulating material to prevent heat loss. This cover, or a part of it, is removed prior to pouring metal into the mold, and is replaced after pouring to insure better control of the cool-down cycle. Thus, when the molten metal enters the mold, it begins to solidify upwardly from the chill block in a manner dictated by the existence of the thermal gradients in the mold caused by the preheating. During solidification of the metal in the mold, the temperatures are reduced to maintain the desired thermal gradients at the interface between the solid "and the liquid metal.

The temperatures involved in this type of procedure may be close to 3000 F. for the graphite susceptor. This high temperature, coupled with the high degree of vacuum normally used in pouring the superalloys frequently causes some of the constituents of the graphite susceptor, the refractory wall, or even the metal to vaporize from the hotter portions of the system and to condense on the cooler portions of the system. This condensation leads to a build up of loose, flaky material on the cooler portions of the system, and frequently these materials will condense at the pour hole so that upon subsequent introduction of the metal, these contaminants will be picked up by the molten metal and will appear in the finished casting. The contamination can be so severe as to cause the casting to be scrapped for dirt and inclusions.

With the foregoing in mind, an object of the present invention is to provide an improved apparatus for casting high temperature alloys in order to avoid contamination by condensed impurities which have been volatilized from the environment of the furnace.

Another object of the invention is to provide an ap- 3,385,346 Patented May 28, 1968 paratus for casting superalloys and the like wherein surfaces are provided within the furnace to preferentially condense any volatilized impurities, and to remove them from the casting zone before the introduction of the molten metal.

Still another object of the invention is to provide a method for casting high temperature alloys to insure that the molten metal remains uncontaminated both during pouring and during the cool-down cycle.

A further explanation of the present invention will be made in conjunction with the attached sheet of drawings in 'which:

FIGURE 1 is a vertical cross-sectional view of a casting assembly incorporating the improvements of the present invention;

FIGURE 2 is a fragmentary cross-sectional view of a prior art apparatus which presented problems now overcome by the improvements of the present invention; and

FIGURE 3 is a fragmentary cross-sectional view of still another prior art device which presented similar problems.

As shown in the drawings:

in FIGURE 1, reference numeral 10 indicates generally a casting apparatus of the type used for the casting of superalloys and the like, the entire furance assembly 10 being normally enclosed within a vacuum chamber (not shown). The furnace 10 includes an outer cylindrical wall 11 disposed on a suitable refractory table 12. Inwardly of the wall 11 there is a refractory sleeve 13 composed of a highly refractory material such as zirconia or the like. Between the sleeve 13 and the wall 11 is an induction coil 14 which provides the heat energy for the furnace. A susceptor 16 composed of graphite or other electrically conductive, refractory material forms the inner Wall of the furnace assembly. The high frequency currents generated by the induction coil 14 cause the susceptor 16 to be heated to extremely high temperatures, frequently approaching 3000 F., so that it radiates heat onto a relatively porous, refractory shell mold 17. The shell mold 17 is generally on the order of to A inch in thickness, and is built up by standard precision investment casting mold making processes. Typically, a wax pattern is dipped in a slurry containing a ceramic such as zirconium oxide, a binder such as collo dal si ica, and a thickener and low temperature binder such as methyl cellulose. The initial layer while still wet is dusted with small particles (40 to 2.00 mesh) of a refractory glass composition as a finely divided high silicon oxide glass containing about 96% silica and a small amount of boric acid together with traces of aluminum, sodium, iron and arsenic. The dusted Wet refractory layer is then moved through a drying oven having a controlled humidity and temperature, so that the coated pattern is dried adiabatically. The steps of dipping, dusting, and adiabatic drying are then repeated using air at progressively lower humidities for succeeding coats. A total of seven coats or so usually builds up the required thickness in mate rial, when the thickness of the individual layers range from about 0.005 to 0.020 inch. After the mold is thus built up on the pattern, the pattern material can be removed by heat, and the green mold is ready for firing. Generally, firing temperatures on the order of 1500 to 1900 F. are used.

The ceramic shell mold 17 includes a gate portion 18 for the introduction of molten metal into the casting cavity generally indicated at reference numeral 19 in the drawings. The cavity 19 terminates in an open ended portion 21 which rests on a chill block 22 composed of copper or other highly heat conductive material. The chill block 22 is made movable vertically with respect to the furnace enclosure by being secured to a rod 23. Movement of the chill block 22 relative to the furnace assembly is desirable during the cool-down cycles so that the desired temperature gradient can be maintained in the metal being solidified in the casting cavity 19.

The top of the furnace assembly is closed off by means of a refractory cover plate 24 having a centrally dispossed aperture 26 therein located above the gate 18 for the purpose of introducing molten metal into the casting cavity 19. Before proceeding with a description of the improvements of the present invention, it would be well to explain the difficulties of the prior art as shown in the drawings of FIGURES 2 and 3, wherein comparable elements have been given the same reference numerals as in FIGURE 1.

One technique for pouring in the prior art, as illustrated in FIGURE 2, consisted in placing a removable cover 28 over the central aperture 26 of the cover plate 24, the cover 28 serving to retain the heat in the furnace assembly during preheating of the mold 17, and also during the cool-down cycle. With the use of this type of system, however, during preheating of the mold 17 and subsequently when the mold has been filled, the extreme heat would cause volatilization of some graphite from the susceptor 16 or some ceramic material from the mold 17. It was found that these volatilized constituents would deposit at the line of contact between the cover 28 and the cover plate 24, forming a flaky type deposit 29, as indicated in FIGURE 2 of the drawings. Then, after preheating, when the cover 28 was removed, the moulten metal poured through the aperture 26, the metal would pick up this flaky deposit 29 and become contaminated. Such contamination would show up as inclusions in the cast structure.

Another pouring technique used in the past has been the use of a ceramic funnel 31 having a flange portion 32 resting on the periphery of the central aperture 26. However, the same problem was presented in that the volatilization of the refractory materials would result in condensation along the conical surface 33 of the funnel 31, leaving the deposit 34. Upon pouring of the metal through the funnel 31, this deposit 34 would be Washed down by the molten metal into the molding cavity 19 and again provide undesirable contamination.

Returning to the showing of FIGURE 1, the present invention overcomes the difficulties incident to the types of structures shown in FIGURES 2 and 3 by providing for selective condensation of the impurities upon surfaces which are at a lower temperature than the remainder of the assembly and which can be withdrawn from proximity to the pour hole during the pouring of the molten metal. In the form of the invention illustrated in FIG- URE l, the improvement takes the form of a combined cover and funnel assembly including a hollow cover member 36 having conical wall portions 37 therein. The cover and the conical wall portions can be made integral with each other, or they can be separate elements mechanically joined together. It has been found that for best results some portions of the removable cover should have angularly inclined surfaces, since these apparently provide the best site for the condensation of the volatilized impurities. Through the use of the angularly disposed condensing surfaces, the impurities are deposited almost entirely upon the surfaces, while leaving the periphery of the pour hole or aperture 26 free from such deposition.

Thus, during preheat of the mold, the impurities condense as a deposit 38 on the conical surface 37 and, when pouring is ready to be commenced, the entire cover 36 is removed, thereby exposing the aperture 26 in a clean, uncontaminated condition. After pouring, the cover 36 is replaced over the aperture 26 to assist in maintaining optimum conditions for the cool-down cycle.

From the foregoing, it will be understood that the method and apparatus for the present invention provide a means for keeping undesirable contaminants, produced by extremely high temperature operation, from being introduced into the molding cavity. It should also be understood that various modifications can be made to the described embodiments without departing from the scope of the present invention.

I claim as my invention:

1. In a casting assembly including a refractory mold within a refractory furnace operating at a temperature sufficiently high to cause volatilization of refractory materials therein, the improvement which comprises acover plate forming a top closure for said assembly and having a centrally disposed pour hole located in line with the axis of the mold positioned within said furance, and a cover removably positioned within said pour hole, said cover including internal surfaces interiorly of said cover and; extending at an angle to said axis whereby said surfaces are shielded from direct radiation from said furnace, thereby providing relatively cooler surfaces for condensation of refractory materials volatilized within said furnace, said cover and internal surface structure being associated in the form of an unitary removable closure structure separate from said IIDOld structure and effective to close said pour hole and removable as a unit.

2. The method of casting high temperature alloys which comprises positioning a ceramic mold within a furnace having an apertured cover plate thereon with the axis of the mold cavity being in line with the axis of the aperture in said cover plate, closing off said aperture with a cover including an internal wall at an angle to said axis, said internal wall being out of the line of direct radiation from said furnace, said cover and internal surface structure being associated in the form of a unitary removable closure structure separate from said mold structure and effective to close said pour hole and removable as a unit, preheating the furnace to a temperature sufiiciently high to volatilize refractory materials therein, and condensing such material on the relatively cooler surfaces of said internal wall, removing said cover from said aperture, pouring molten metal through said aperture and into said mold, and replacing said cover in said aperture after the pouring has been completed.

References Cited UNITED STATES PATENTS 1,839,106 12/1931 LOth 164348 2,806,271 9/1957 Operhall 164-65 3,188,702 6/1965 Smurthweite 164337 3,200,455 8/1965 Operhall et a1. 164-421 I. SPENCER OVERHOLSER, Primary Examiner.

V. RISING, Assistant Examiner. 

